Conventional alkaline electrochemical cells have an anode comprising zinc and a cathode comprising manganese dioxide. The cell is typically formed of a cylindrical outer housing. The fresh cell has an open circuit voltage (EMF) of about 1.5 volt and typical average running voltage of between about 1.0 to 1.2 Volt in medium drain service (100 to 300 milliamp). The cylindrical housing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap assembly with insulating grommet and negative terminal end cap is inserted into the housing open end. The open end is closed by crimping the housing edge over an edge of the insulating plug and radially compressing the housing around the insulating plug to provide a tight seal. The insulating grommet electrically insulates the negative end cap from the cell housing. A portion of the cell housing at the opposing closed end forms the positive terminal.
A problem associated with design of various electrochemical cells, particularly alkaline cells, is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally near the point of complete exhaustion of the cell's useful capacity. Electrochemical cells, particularly alkaline cells, are conventionally provided with rupturable diaphragms or rupturable membranes within the end cap assembly. The rupturable diaphragm or membrane may be formed within a plastic insulating member as described, for example, in U.S. Pat. No. 3,617,386.
The prior art discloses rupturable vent membranes, which are integrally formed as thinned areas within the insulating disk included within the end cap assembly. Such vent membranes can be oriented such that they lie in a plane perpendicular to the cell's longitudinal axis, for example, as shown in U.S. Pat, No. 5,589,293, or they may be oriented so that they are slanted in relation to the cell's longitudinal axis as shown in U.S. Pat. No. 4,227,701. U.S. Pat. No. 6,127,062 discloses an insulating sealing disk and an integrally formed rupturable membrane, which is oriented vertically, that is, parallel to the cell's central longitudinal axis. When the gas pressure within the cell rises to a predetermined level the membrane ruptures thereby releasing the gas pressure to the external environment through apertures in the end cap.
Other types of vents are disclosed in the art for relieving gas pressure within an electrochemical cell. One such vent is a reseatable rubber plug, which has been used effectively in connection with small flat rectangular shaped nickel metal hydride rechargeable cells. One such rechargeable battery with the reseatable rubber plug vent is a 7/5-F6 size nickel metal hydride rechargeable battery available commercially as battery model GP14M145 manufactured by Gold Peak Batteries, Hong Kong. The rubber plug is physically compressed to sit tightly within a beveled aperture within a cavity or seat in the cell's end cap assembly. When the cell's internal gas pressure reaches a predetermined level, the plug lifts off its seat thereby letting gas to escape through the underlying aperture. The plug reseats itself when the gas pressure within the cell returns to normal.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, small amount of polymeric binders, for example polyethylene binder and other additives, such as titanium-containing compounds can be added to the cathode.
The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable, since it has a high density and high purity. The electrical conductivity (resistivity) of EMD is fairly low. An electrically conductive material is added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector in conventional cylindrical alkaline cells. Suitable electrically conductive additives can include, for example, graphite, graphitic material, conductive carbon powders, such as carbon blacks, including acetylene blacks. Preferably the conductive material comprises flaky crystalline natural graphite, or flaky crystalline synthetic graphite, including expanded or exfoliated graphite or graphitic carbon nanofibers and mixtures thereof.
There are small size rectangular shaped rechargeable batteries now available, which are used to power small electronic devices such as MP3 audio players and mini disk (MD) players. These batteries are typically in the shape of a small cuboid (rectangular parallelepiped) somewhat the size of a pack of chewing gum. The term “cuboid” as used herein shall mean its normal geometrical definition, namely, a “rectangular parallelepiped”. Such batteries, for example, can be in the form of replaceable rechargeable nickel metal hydride (NiMH) size F6 or 7/5F6 size cuboids in accordance with the standard size for such batteries as set forth by the International Electrotechnical Commission (IEC). The F6 size has a thickness of 6.0 mm, width of 17.0 mm and length of 35.7 mm (without label). There is a version of the F6 size wherein the length can be as great as about 48.0 mm. The 7/5-F6 size has thickness of 6.0 mm, width of 17.0 mm, and length of 67.3 mm. According to the IEC standard, allowed deviation for the 7/5-F6 size in thickness is +0 mm, −0.7 mm, in width is +0 mm, −1 mm, and in length is +0, −1.5 mm. The average running voltage of the F6 or 7/5F6 NiMH rechargeable batteries when used to power miniature digital audio players such as an MP3 audio player or mini disk (MD) players is between about 1.1 and 1.4 volt typically about 1.12 volt.
When used to power the mini disk (MD) player the battery is drained at a rate of between about 200 and 250 milliAmp. When used to power a digital audio MP3 player the battery is drained typically at a rate of about 100 milliAmp.
It would be desirable to have a small flat alkaline battery of the same size and shape as small size cuboid shaped (rectangular parallelepiped) nickel metal hydride batteries, so that the small alkaline size battery can be used interchangeably with the nickel metal hydride battery to power small electronic devices such as mini disk or MP3 players.
It would be desirable to use a primary (nonrechargeble) alkaline battery, preferably a zinc/MnO2 alkaline battery as a replacement for small rectangular shaped rechargeable batteries, particularly small size nickel metal hydride rechargeable battery.
However, a particular problem associated with the design of rectangular (cuboid) shaped primary Zn/MnO2 alkaline battery is that of the tendency of the electrodes to swell during cell discharge. Both anode and cathode swells during discharge.
For a given housing wall thickness, it will be appreciated that a rectangular shaped cell housing is less able to withstand a given increase in cell internal pressure (due to gassing and cathode expansion) than a cylindrical shaped housing of comparable size and volume. This is due to the significantly higher circumferential stress (hoop stress) imposed on a rectangular (cuboid) shaped housing than on a similar size cylindrical housing for any given pressure and housing wall thickness. The problem of bulging or swelling associated with rectangular shaped cells can be overcome by significantly increasing the wall thickness of the housing. However, a significant increase in housing wall thickness can result in significant decrease in available volume for anode and cathode materials for rectangular cells having small overall thickness, e.g. under about 10 mm. The added wall thickness adds to the cost of manufacture of the cell. In this regard it is desirable to keep the housing wall thickness below about 0.50 mm, preferably less than about 0.47 mm.
Thus it is desired to design a small flat (nonrechargeable) alkaline cell, such as an F6 or 7/5-F6 size cell having a rectangular (cuboid) shaped housing, but yet with small housing wall thickness, wherein the housing does not significantly bulge or swell during normal cell usage.
It is desired that such rectangular cell be used as a replacement for a same size flat nickel metal hydride rechargeable cell.